https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96945
Bug ID: 96945
Summary: optimizations regression when defaulting copy
constructor
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: federico.kircheis at gmail dot com
Target Milestone: ---
While toying with a piece of code, I've noticed that the code did not get
optimized as expected.
All snippets where compiled with -O3.
A)
----
#include <vector>
struct c {
};
void foo(){
std::vector<c> vi = {c(),c(),c()};
}
----
gets compiled to: https://godbolt.org/z/s7YaEf
----
foo():
sub rsp, 24
mov edi, 3
call operator new(unsigned long)
mov esi, 3
mov rdi, rax
movzx eax, WORD PTR [rsp+13]
mov WORD PTR [rdi], ax
movzx eax, BYTE PTR [rsp+15]
mov BYTE PTR [rdi+2], al
add rsp, 24
jmp operator delete(void*, unsigned long)
----
Adding and defaulting the constructors produces even more optimized code (the
whole vector is optimized out(!): https://godbolt.org/z/E4GT9x
B)
----
#include <vector>
struct c {
c() = default;
c(const c&) =default;
c(c&&) = default;
};
void foo(){
std::vector<c> vi = {c(),c(),c()};
}
----
----
foo():
ret
----
Adding and defaulting the constructors, except the move constructor produces
the same code as A): https://godbolt.org/z/ch71fb
B)
----
#include <vector>
struct c {
c() = default;
c(const c&) =default;
c(c&&) = default;
};
void foo(){
std::vector<c> vi = {c(),c(),c()};
}
----
If the copy or default constructor is implemented and not defaulted, then the
code is optimized as B): https://godbolt.org/z/v8E37b,
https://godbolt.org/z/v3EY69, #include <vector>
struct c {
c() {};
};
void foo(){
std::vector<c> vi = {c(),c(),c()};
}
C)
----
#include <vector>
struct c {
c() = default;
c(const c&) {};
};
void foo(){
std::vector<c> vi = {c(),c(),c()};
}
----
D)
----
#include <vector>
struct c {
c() = default;
c(const c&) {};
c(c&&) = default;
};
void foo(){
std::vector<c> vi = {c(),c(),c()};
}
----
E)
----
#include <vector>
struct c {
c() {}
};
void foo(){
std::vector<c> vi = {c(),c(),c()};
}
----
While ideally the code for those cases is equivalent (as c has no state and all
snippets are functionally equivalent), I would have expected the class with
compiler-defined operators have the best codegen, followed by the class with
defaulted operators, and last the class with a non-defaulted implementation.
Strangely all constructor calls of `c` are always optimized away, but depending
how the class is defined g++ does or does not optimize the whole vector away.
